The present invention relates to a transparent substrate used for an area light emitting device, method for making the same, and an area light emitting device including the same, and liquid crystal display device with the area light emitting device. The transparent substrate has a light incidence surface and a light exit surface opposite to the light incidence surface and includes a means for emitting the incident light towards the outside.
A bottom emission type area light emitting device is conventionally known. The light source includes a transparent substrate with a light incidence surface and a light exit surface opposite to the light incidence surface, and a light emitting element such as organic electroluminescence device formed on the light incidence surface of the substrate. Light emitted from the organic electroluminescence device is introduced into the transparent substrate and the incident light is emitted from the light exit surface to the outside.
This type of area light emitting device cannot emit the entire amount of light, which is generated by the light emitting element and is emitted to the transparent substrate, to the outside. This is because the light, other than those incident light rays on the light exit surface with an incident angle smaller than the critical angle, is reflected on the light exit surface due to the difference in both indices. The critical angle on the light exit surface is defined by refractive indices of the transparent substrate and the outer atmosphere (air in general) The light reflected on the light exit surface can be emitted outside of the device through the edge of the transparent substrate, can be attenuated after repeated reflection within the substrate, or can be attenuated by propagation into the area light emitting device, so that it would not be emitted from the light exit surface to the outside.
In order to take out the light that is difficult to emit from the light exit surface, a technique has been proposed to form a light scattering portion on the light exit surface of the transparent substrate (Japanese Laid-Open Patent Publication 8-83688). According to the publication, the light, which cannot be taken out from the light exit surface, is scattered at the light scattering portion to enable a portion of the light to emit from a planar light exit surface of a transparent substrate.
Further, an area light emitting device is required in general to have high luminance in a specific direction on the light exit surface.
For example, in an area light emitting device assembled in a liquid crystal display device, it is necessary that the amount of light emitted in the direction perpendicular to the light exit surface be larger than the amount of light emitted in other directions.
Accordingly, it is necessary that the transparent substrate emits light, which entered from the area light emitting device, at a high rate from the light exit surface (improved light extraction efficiency) and that the luminance in a specific direction at the light exit surface is high (improved light usage).